Re: [PATCH v4 07/12] mm: enable lazy_mmu sections to nest

[Alexander Gordeev <agordeev@xxxxxxxxxxxxx>]

On Thu, Nov 06, 2025 at 10:51:43AM +0000, Kevin Brodsky wrote:
> On 05/11/2025 16:12, Alexander Gordeev wrote:
> > On Wed, Nov 05, 2025 at 02:19:03PM +0530, Ritesh Harjani wrote:
> >>> + * in_lazy_mmu_mode() can be used to check whether the lazy MMU mode is
> >>> + * currently enabled.
> >>>   */
> >>>  #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
> >>>  static inline void lazy_mmu_mode_enable(void)
> >>>  {
> >>> - arch_enter_lazy_mmu_mode();
> >>> + struct lazy_mmu_state *state = &current->lazy_mmu_state;
> >>> +
> >>> + VM_WARN_ON_ONCE(state->nesting_level == U8_MAX);
> >>> + /* enable() must not be called while paused */
> >>> + VM_WARN_ON(state->nesting_level > 0 && !state->active);
> >>> +
> >>> + if (state->nesting_level++ == 0) {
> >>> +         state->active = true;
> >>> +         arch_enter_lazy_mmu_mode();
> >>> + }
> >>>  }
> >> Some architectures disables preemption in their
> >> arch_enter_lazy_mmu_mode(). So shouldn't the state->active = true should
> >> happen after arch_enter_lazy_mmu_mode() has disabled preemption()? i.e.
> > Do you have some scenario in mind that could cause an issue?
> > IOW, what could go wrong if the process is scheduled to another
> > CPU before preempt_disable() is called?
> 
> I'm not sure I understand the issue either.
> 
> >>   static inline void lazy_mmu_mode_enable(void)
> >>   {
> >>  - arch_enter_lazy_mmu_mode();
> >>  + struct lazy_mmu_state *state = &current->lazy_mmu_state;
> >>  +
> >>  + VM_WARN_ON_ONCE(state->nesting_level == U8_MAX);
> >>  + /* enable() must not be called while paused */
> >>  + VM_WARN_ON(state->nesting_level > 0 && !state->active);
> >>  +
> >>  + if (state->nesting_level++ == 0) {
> >>  +         arch_enter_lazy_mmu_mode();
> >>  +         state->active = true;
> >>  + }
> >>   }
> >>
> >> ... I think it make more sense to enable the state after the arch_**
> >> call right.
> > But then in_lazy_mmu_mode() would return false if called from
> > arch_enter_lazy_mmu_mode(). Not big problem, but still..
> 
> The ordering of nesting_level/active was the way you expected in v3, but
> the conclusion of the discussion with David H [1] is that it doesn't
> really matter so I simplified the ordering in v4 - the arch hooks
> shouldn't call in_lazy_mmu_mode() or inspect lazy_mmu_state.
> arch_enter()/arch_leave() shouldn't need it anyway since they're called
> once per outer section (not in nested sections). arch_flush() could
> potentially do something different when nested, but that seems unlikely.
> 
> - Kevin
> 
> [1]
> https://lore.kernel.org/all/af4414b6-617c-4dc8-bddc-3ea00d1f6f3b@xxxxxxxxxx/

I might be misunderstand this conversation, but it looked to me as a discussion
about lazy_mmu_state::nesting_level value, not lazy_mmu_state::active.

I do use in_lazy_mmu_mode() (lazy_mmu_state::active) check from the arch-
callbacks. Here is the example (and likely the only case so far) where it hits:

static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
                                      void *_data)
{
        lazy_mmu_mode_pause();
        ...
        if (likely(pte_none(ptep_get(ptep)))) {

                /* Here set_pte() checks whether we are in lazy_mmu mode */
                set_pte_at(&init_mm, addr, ptep, pte);  <--- calls set_pte()
                data->pages[index] = NULL;
        }
        ...
        lazy_mmu_mode_resume();
        ...
}

So without in_lazy_mmu_mode() check above the arch-specific set_pte()
implementation enters a wrong branch, which ends up in:

[  394.503134] Call Trace:
[  394.503137]  [<00007fffe01333f4>] dump_stack_lvl+0xbc/0xf0 
[  394.503143]  [<00007fffe010298c>] vpanic+0x1cc/0x418 
[  394.503149]  [<00007fffe0102c7a>] panic+0xa2/0xa8 
[  394.503154]  [<00007fffe01e7a8a>] check_panic_on_warn+0x8a/0xb0 
[  394.503160]  [<00007fffe082d122>] end_report+0x72/0x110 
[  394.503166]  [<00007fffe082d3e6>] kasan_report+0xc6/0x100 
[  394.503171]  [<00007fffe01b9556>] ipte_batch_ptep_get+0x146/0x150 
[  394.503176]  [<00007fffe0830096>] kasan_populate_vmalloc_pte+0xe6/0x1e0 
[  394.503183]  [<00007fffe0718050>] apply_to_pte_range+0x1a0/0x570 
[  394.503189]  [<00007fffe07260fa>] __apply_to_page_range+0x3ca/0x8f0 
[  394.503195]  [<00007fffe0726648>] apply_to_page_range+0x28/0x40 
[  394.503201]  [<00007fffe082fe34>] __kasan_populate_vmalloc+0x324/0x340 
[  394.503207]  [<00007fffe076954e>] alloc_vmap_area+0x31e/0xbf0 
[  394.503213]  [<00007fffe0770106>] __get_vm_area_node+0x1a6/0x2d0 
[  394.503218]  [<00007fffe07716fa>] __vmalloc_node_range_noprof+0xba/0x260 
[  394.503224]  [<00007fffe0771970>] __vmalloc_node_noprof+0xd0/0x110 
[  394.503229]  [<00007fffe0771a22>] vmalloc_noprof+0x32/0x40 
[  394.503234]  [<00007fff604eaa42>] full_fit_alloc_test+0xb2/0x3e0 
[test_vmalloc] 
[  394.503241]  [<00007fff604eb478>] test_func+0x488/0x760 [test_vmalloc] 
[  394.503247]  [<00007fffe025ad68>] kthread+0x368/0x630 
[  394.503253]  [<00007fffe01391e0>] __ret_from_fork+0xd0/0x490 
[  394.503259]  [<00007fffe24e468a>] ret_from_fork+0xa/0x30 

I could have cached lazy_mmu_state::active as arch-specific data
and check it, but then what is the point to have it generalized?

Thanks!